Automobile vibration-isolating rubber composition and automobile vibration-isolating device

ABSTRACT

Provided is an automobile vibration-isolating rubber composition, characterized in that a rubber component comprises from 10 to 80% by weight of a polybutadiene rubber which is obtained through the polymerization using a rare-earth element catalyst and which has a cis content of 97% or more, and from 90 to 20% by weight of at least one type of a rubber selected from a natural rubber, a synthetic polyisoprene rubber and a polystyrene-butadiene copolymer rubber. The automobile vibration-isolating rubber obtained by vulcanizing the rubber composition of the present invention has a low dynamic factor compared to the conventional product, making it possible to decrease a room noise caused by a road noise.

BACKGROUND OF THE INVENTION

The present invention relates to a rubber composition which ispreferably used as an automobile vibration-isolating rubber, and to anautomobile vibration-isolating device having a strut mount obtained fromthe same.

A natural rubber composition or a rubber composition comprising a blendof a natural rubber and a polybutadiene rubber or apolystyrene-butadiene copolymer rubber has been hitherto used as anautomobile vibration-isolating rubber of a strut mount, an engine mount,a body mount, a bush or the like.

In order to reduce a passenger compartment noise which is caused by roadnoise emanating from an automobile road surface, it is advisable that ina strut mount, for example, a dynamic spring constant in a highfrequency region of from 100 to 300 Hz be decreased and a dynamic factor(dynamic-to-static modulus ratio) be closer to 1. Therefore, in theabove-mentioned vibration-isolating rubber, a rubber compositioncomprising a natural rubber and a polybutadiene rubber has been recentlyoften selected and used. However, since the polybutadiene rubber whichhas been used to date is produced in the presence of a cobalt catalystor a nickel catalyst and has a cis content of less than 97%, an effectof a decrease in the dynamic factor is not sufficient as will be laterdescribed, and a satisfactory decrease in the dynamic factor has been indemand.

Especially, as an automobile weight is reduced to meet social demandsfor energy saving and exhaust gas control in recent years, there is agrowing tendency that a rolling resistance of a tire is decreased or aweight of a tire is reduced. However, a tire having the low rollingresistance or the light-weight tire generally tends to increase the roadnoise. Thus, the decrease in the dynamic factor has been in high demand.

SUMMARY OF THE INVENTION

The present invention is to provide an automobile vibration-isolatingrubber composition of which the dynamic factor is decreased more than asusual.

The present inventors have assiduously conducted investigations to solvethe above-mentioned problems, and have consequently found that thepredetermined object can be achieved especially by selecting a specificpolybutadiene rubber having a high cis content and forming a rubbercomposition containing a specific amount of this polybutadiene rubber.This finding has led to the completion of the present invention.

That is, in the automobile vibration-isolating rubber composition of thepresent invention, a rubber component, which is to be vulcanized,comprises from 10 to 80% by weight of a polybutadiene rubber which isobtained through the polymerization using a rare-earth element catalystand which has a cis content of 97% or more, and from 90 to 20% by weightof at least one type of a rubber selected from a natural rubber, asynthetic polyisoprene rubber and a polystyrene-butadiene copolymerrubber.

In this construction, a vibration-isolating rubber composition having adynamic factor which is lower than that of a conventional material canbe designed. When this composition is used in an automobilevibration-isolating rubber, it is possible to decrease a automobilepassenger compartment noise caused by road noise.

The automobile vibration-isolating rubber composition of the presentinvention preferably contains carbon black having a nitrogen adsorptionspecific surface area (hereinafter referred to as "N₂ SA") of 40 m² /gor less and a dibutyl phthalate oil absorption (hereinafter referred toas "DBP oil absorption") of 80 ml/100 g or more, and a compoundrepresented by formula (1), in addition to the rubber component.##STR1##

The compound of formula (1) is a coupling agent by which to couple arubber and carbon black. The specific coupling agent and the specificcarbon black are incorporated with the above-mentioned rubber component,making it possible to extremely increase the effect of the decrease inthe dynamic factor.

The automobile vibration-isolating device of the present invention has astrut mount obtained from the above-mentioned automobilevibration-isolating rubber composition in an automobile havinglight-weight radial tires for a passenger car (hereinafter referred toas "light-weight tires") or radial tires for a passenger car having lowrolling resistances (hereinafter referred to as "low rolling resistancetires").

Since the vibration-isolating rubber composition of the presentinvention has the excellent effect of the decrease in the dynamic factoras stated above, the vibration-isolating device having the strut mountobtained from the rubber composition in the present invention does notincrease the road noise even when the automobile has the light-weighttires or the low rolling resistance tires. Especially when theabove-mentioned specific carbon black and coupling agent are used incombination, the dynamic factor of the strut mount is extremely low, andthe effect of the decrease in the load noise is therefore quiteexcellent. Accordingly, the automobile vibration-isolating device of thepresent invention can decrease the weight of the tire or decrease thefuel consumption without increasing the road noise. That is, it can meetthe social demands for reduction of the automobile weight and the energysaving without increasing the room noise in running.

In the vibration-isolating device of the present invention, the strutmount may be a liquid-filled strut mount which has a liquid chamberwherein the vibration-isolating rubber constitutes a part of a chamberwall and which damps the vibration through an elasticity of thevibration-isolating rubber and a liquid-flowing effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing an example of a strut mount obtainedfrom an automobile vibration-isolating rubber composition of the presentinvention.

FIG. 2 is a sectional view showing an example of a liquid-filled strutmount obtained from an automobile vibration-isolating rubber compositionof the present invention.

FIG. 3 is a sectional view of structures of a light-weight tire and ageneral radial tire.

FIG. 4 is a graph showing a relationship between a frequency and a soundpressure level in an actual car test of strut mounts obtained fromvibration-isolating rubber compositions in Example 2 and ComparativeExample 1.

DETAILED DESCRIPTION OF PREFERABLE EMBODIMENTS

In the automobile vibration-isolating rubber composition of the presentinvention, the polybutadiene rubber containing as high as 97% or more ofa cis which is a rubber component of a vulcanized rubber can usually beformed through the polymerization using a rare-earth element catalyst.As the rare-earth element catalyst, a neodymium catalyst is preferablyused. For example, a polybutadiene rubber having a cis content of 98%(Neocis BR, made by Enichem Elastmeri, and Buna CB, made by Bayer AG)which is produced in the presence of a neodymium catalyst is preferable.

The amount of the polybutadiene rubber having the cis content of 97% ormore in the rubber component is between 10 and 80% by weight, preferablybetween 20 and 40% by weight. When this amount is less than 10% byweight, the effect of the decrease in the dynamic factor isinsufficient, and a compression set and a fatigue resistance tend todecrease. When the amount is more than 80% by weight, a strength and anelongation are notably decreased, and a mill processability is poor.Thus, it is not practical.

As the other rubber of the rubber component with which the polybutadienerubber having the cis content of 97% or more is blended, a naturalrubber, a synthetic polyisoprene rubber and a polystyrene-butadienecopolymer rubber are used either singly or in combination. The amount ofthe other rubber in the rubber component is between 90 and 20% byweight, preferably between 80 and 60% by weight.

The compound of formula (1) is preferablyN,N'-bis(2-methyl-2-nitropropyl)-1,6-diaminohexane. The amount of thiscoupling agent is preferably between 0.5 and 4.0 parts by weight per 100parts by weight of the rubber component.

Carbon black, as stated above, has preferably N₂ SA of 40 m² /g or lessand a DBP oil absorption of 80 ml/100 g or more. When the N₂ SA ishigher than 40 m² /g, the dynamic factor is increased. When the DBP oilabsorption is less than 80 ml/100 g, a static spring constant isdecreased if the amount of the carbon black is fixed, and the dynamicspring constant is increased if the static spring constant is fixed.Thus, in both cases the effect of the decrease in the dynamic factor isinsufficient. The preferable amount of such a specific carbon black isbetween 20 and 80 parts by weight per 100 parts by weight of the rubbercomponent.

The automobile vibration-isolating rubber composition of the presentinvention can contain, besides the above-mentioned components, variousknown additives which are incorporated into the usual rubbercomposition, namely, the usual carbon black other than theabove-mentioned specific carbon black, zinc oxide, stearic acid, sulfur,a vulcanization accelerator, a softening agent and an antioxidant asrequired.

The automobile vibration-isolating rubber composition of the presentinvention can be obtained by blending the polybutadiene rubber obtainedthrough the polymerization in the presence of the rare-earth elementcatalyst and having the cis content of 97% or more with theabove-mentioned other diene rubber such as the natural rubber or thelike to form the rubber component, adding the above-mentioned variousadditives, preferably, the specific carbon black and the specificcoupling agent to the rubber component, and kneading the mixture in ausual manner. When this rubber composition is vulcanized under knownvulcanization conditions, an automobile vibration-isolating rubber canbe formed.

The thus-obtained automobile vibration-isolating rubber has the higheffect of the decrease in the dynamic factor and can reduce the roomnoise which is caused by the road noise given from the automobile roadsurface.

Accordingly, the automobile vibration-isolating device of the presentinvention which has the strut mount obtained from thisvibration-isolating rubber composition can prevent the above-mentionedincrease in the road noise even if the automobile is fitted with thelight-weight tires or the low rolling resistance tires. That is, theincrease in the road noise with the conventional vibration-isolatingdevice when the automobile is fitted with the light-weight tires or thelow rolling resistance tires can be offset upon using thevibration-isolating device of the present invention, and the road noisecan be more reduced. Especially when using the rubber compositioncontaining the above-mentioned specific coupling agent and specificcarbon black, such an effect in the decrease of the road noise is quiteexcellent.

The light-weight tire refers to, for example, a radial tire of which theweight is reduced by 10% or more compared to a conventional radial tirefor a passenger car (hereinafter referred to as "a general tire"). Thisreduction of the weight is conducted, for example, by forming a carcasswith 1 ply and reducing thicknesses of a tread, a side portion and abead by from 10 to 20%.

The low rolling resistance tire refers to a low fuel consumption tirewhich is a radial tire in which a coefficient of a rolling resistance[=(rolling resistance/longitudinal load)×100] is 1.0 or less while thatof a general tire is 1.5 or more and the rolling resistance is thus low.

An example of the strut mount obtained from the rubber composition ofthe present invention is as shown in FIG. 1. This strut mount 10comprises a main body fitting 16, an installation fitting 20 installedon the automobile main body with a bolt 18, and an vibration-isolatingrubber 22 which is disposed between the main body fitting 16 and theinstallation fitting 20 to communicate these fittings resiliently. Themain body fitting 16 has a cylindrical body, into which a top of a strutrod 12 is introduced, and is mounted on the top through a bearing 14.The vibration-isolating rubber 22 is formed by vulcanizing theautomobile vibration-isolating rubber composition of the presentinvention.

As the strut mount, a liquid-filled strut mount which has a liquidchamber wherein the vibration-isolating rubber constitutes a part of thechamber wall and which damps the vibration by the elasticity of thevibration-isolating rubber and the liquid-flowing effect can also beused.

FIG. 2 shows an example of a liquid-filled strut mount 50. This strutmount 50 comprises a main body fitting 54, an installation fitting 58installed on the automobile main body with a bolt 56, anvibration-isolating rubber 60 which is disposed between the main bodyfitting 54 and the installation fitting 58 to communicate these fittingsresiliently, and a diaphragm 62 mounted between the main body fitting 54and the installation fitting 58. The main body fitting 54 has acylindrical body, into which a top of a strut rod 12 is introduced, andis mounted on the top through a bearing 52. A liquid chamber 64 filledwith a liquid is formed within the mount by the vibration-isolatingrubber 60 and the diaphragm 62. This liquid chamber 64 is divided intotwo chambers which are vertically communicated through a hollow discpartition plate 66. The vibration-isolating rubber 60 is formed byvulcanizing the automobile vibration-isolating rubber composition of thepresent invention.

When the liquid-filled strut mount having the above-mentioned structureis used, it is possible to further decrease the dynamic factor by theeffect of the decrease in the dynamic factor with thevibration-isolating rubber and the liquid-flowing effect.

The present invention is illustrated more specifically by referring tothe following Examples and Comparative Examples.

EXAMPLES 1 TO 9 AND COMPARATIVE EXAMPLES 1 AND 2

The components shown in Table 1 were blended, and the followingadditives were added thereto. The mixture was kneaded by a usual method.In this manner, a total of 11 rubber compositions shown in Examples 1 to9 and Comparative Examples 1 and 2 were prepared.

    ______________________________________                                                        parts by weight                                               ______________________________________                                        rubber component  100 (refer to Table 1)                                      carbon black      (refer to Table 1)                                          coupling agent    (refer to Table 1)                                          aromatic oil      3                                                           zinc oxide        5                                                           stearic acid      1                                                           antioxidant (6C)  2                                                           sulfur            3.5                                                         vulcanization accelerator (CBS)                                                                 1                                                           ______________________________________                                    

In Table 1, "Nd--BR" and "Co--BR" are as follows.

Nd--BR: polybutadiene rubber formed through the polymerization in thepresence of a neodymium catalyst and having a cis content of 98% (NeocisBR60, made by Enichem Elastomeri)

Co--BR: polybutadiene rubber formed through the polymerization in thepresence of a neodymium catalyst and having a cis content of 96% (UbePol 150B, made by Ube Industries, Ltd.)

The N₂ SA and the DBP oil absorption of carbon blacks A to D are asfollows.

    ______________________________________                                        Carbon black A:                                                                              N.sub.2 SA = 26 m.sup.2 /g, DBP oil absorp-                                   tion = 87 ml/100 g                                             Carbon black B:                                                                              N.sub.2 SA = 28 m.sup.2 /g, DBP oil absorp-                                   tion = 154 ml/100 g                                            Carbon black C:                                                                              N.sub.2 SA = 42 m.sup.2 /g, DBP oil absorp-                                   tion = 115 ml/100 g                                            Carbon black D:                                                                              N.sub.2 SA = 27 m.sup.2 /g, DBP oil absorp-                                   tion = 68 ml/100 g                                             ______________________________________                                    

N,N'-bis(2-methyl-2-nitropropyl)-1,6-diaminohexane was used as acoupling agent.

This rubber composition was then vulcanized at a vulcanizationtemperature of 150° C. for 25 minutes to form a test piece having adiameter of 50 mm and a thickness of 25 mm. This test piece was measuredfor dynamic properties (static spring constant, dynamic spring constantand dynamic factor). Further, the rubber composition was vulcanized at150° C. for 15 minutes to form a test sample. This test sample wasmeasured for properties (rubber hardness, tensile strength andelongation) and a mill processability. The results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________            Ex. 1                                                                              Ex. 2                                                                              Ex. 3                                                                              Ex. 4                                                                              Ex. 5                                                                              Ex. 6                                        __________________________________________________________________________    Rubber                                                                        component                                                                     Natural rubber                                                                        80   80   60   20   80   80                                           Nd--Br  20   20   40   80   20   20                                           Co--BR  --   --   --   --   --   --                                           Carbon black A                                                                        60   --   60   60   60   60                                           B       --   50   --   --   --   --                                           C       --   --   --   --   --   --                                           D       --   --   --   --   --   --                                           Coupling agent                                                                          1.5                                                                                1.5                                                                                1.5                                                                                1.5                                                                                4.0                                                                                0.5                                        Hardness of                                                                           67   67   69   70   68   65                                           rubber (JIS-A)                                                                Tensile   20.9                                                                               20.5                                                                               16.3                                                                               11.4                                                                               20.4                                                                               20.4                                       strength (MPa)                                                                Elongation (%)                                                                        390  330  280  250  370  420                                          Mill    ∘                                                                      ∘                                                                      ∘                                                                      Δ                                                                            ∘                                                                      ∘                                processability                                                                Static spring                                                                         635  643  654  662  650  611                                          constant (N/mm)                                                               Dynamic spring                                                                        920  881  936  927  878  950                                          constant (N/mm)                                                               Dynamic factor                                                                           1.45                                                                               1.37                                                                               1.43                                                                               1.40                                                                               1.35                                                                               1.55                                      Effect of                                                                             ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                             decrease in                                                                   dynamic factor                                                                __________________________________________________________________________            Ex. 7 Ex. 8 Ex. 9 CEx. 1                                                                              CEx. 2                                        __________________________________________________________________________    Rubber                                                                        component                                                                     Natural rubber                                                                        80    80    80    100   80                                            Nd--Br  20    20    20    --    --                                            Co--BR  --    --    --    --    20                                            Carbon black A                                                                        --    60    --    60    60                                            B       --    --    --    --    --                                            C       50    --    --    --    --                                            D       --    --    65    --                                                  Coupling agent                                                                          1.5 --    --    --    --                                            Hardness of                                                                           67    65    63    64    64                                            rubber (JIS-A)                                                                Tensile   21.3                                                                                20.9                                                                                18.5                                                                                23.5                                                                                19.6                                        strength (MPa)                                                                Elongation (%)                                                                        370   420   410   450   430                                           Mill    ∘                                                                       ∘                                                                       ∘                                                                       ∘                                                                       ∘                                 processability                                                                Static spring                                                                         632   602   584   590   592                                           constant (N/mm)                                                               Dynamic spring                                                                        1023  988   969   1067  1040                                          constant (N/mm)                                                               Dynamic factor                                                                           1.62                                                                                1.64                                                                                1.66                                                                                1.81                                                                                1.76                                       Effect of                                                                             ∘                                                                       ∘                                                                       ∘                                                                       x     x                                             decrease in                                                                   dynamic factor                                                                __________________________________________________________________________

Ex.--Example, CEx.--Comparative Example

Methods of measuring the above-mentioned properties are as follows.

Static spring constant:

The above-mentioned test piece was compressed with a deflection amountof from 0 to 5 mm twice at a crosshead speed of 10 mm/min using ameasuring device Tensilon (manufactured by Orientech K. K.). Aload-deflection curve in the second compression was drawn, and thestatic spring constant was calculated according to the followingequation.

    Static spring constant (N/mm)=(w.sub.2 -w.sub.1)/(δ.sub.2 -δ.sub.1)

wherein

w₁ is a load when a deflection amount (δ₁) is 1.3 mm,

and

w₂ is a load when a deflection amount (δ₂) is 3.8 mm.

Dynamic spring constant:

The test piece was compressed with a preload of 780 N, a frequency of100 Hz and an amplitude of ±0.05 mm using a measuring device DynamicSERVO (manufactured by Saginomiya Seisakusho Inc.), and the dynamicspring constant of the test piece was calculated according to the methodprescribed in JIS K6394.

Dynamic factor:

The dynamic factor was evaluated in terms of a ratio of the dynamicspring constant to the static spring constant (dynamic springconstant/static spring constant) as measured by the above-mentionedmethods. The smaller value of this dynamic factor is more advantageousto reduce the automobile room noise.

Hardness, tensile strength and elongation:

These properties were measured in accordance with JIS K6301.

Mill processability:

The mill processability was evaluated in terms of a windability onto a10-inch roll according to the following standard. ∘: good millprocessability Δ: poor mill processability

As is apparent from Table 1, the rubber compositions in Examples 1 to 9which were formed by blending the natural rubber with the polybutadienerubber having the cis content of 98% and obtained through thepolymerization in the presence of the neodymium catalyst exhibited thelow dynamic factor and the large effect of the decrease in the dynamicfactor compared to the rubber compositions in Comparative Examples 1 and2. Especially, the rubber compositions in Examples 1 to 6 whichcontained both the specific coupling agent and the specific carbon black(A or B) exhibited the very low dynamic factor and the excellent effectof the decrease in the dynamic factor.

The strut mounts shown in FIG. 1 were formed using the rubbercompositions in Examples 1 and 2 and Comparative Example 1, and weremeasured for the dynamic properties. The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                                           Compara-                                   Type of a rubber                   tive                                       composition    Example 1 Example 2 Example 1                                  ______________________________________                                        Static spring constant                                                                       733       749       699                                        (N/mm)                                                                        Dynamic spring constant                                                                      1273      1163      1510                                       (N/mm)                                                                        Absolute spring constant                                                                     1549      1387      1946                                       (N/mm)                                                                        Dynamic                                                                              Dynamic spring                                                                            1.74      1.60    2.16                                     factor constant/static                                                               spring constant                                                               Absolute spring                                                                           2.12      1.85    2.78                                            constant/static                                                               spring constant                                                        ______________________________________                                    

The static spring constant and the dynamic spring constant in Table 2were measured under the same conditions as those in Table 1. That is,with respect to the static spring constant, a load of from 0 to 29,000 Nwas exerted three times on the strut mount in the axial direction of therod 12 in FIG. 1 at a crosshead speed of 20 mm/min. The static springconstant was calculated with respect to a load of from 1,755 N to 5,265N in a load-deflection curve in the third compression. Further, thedynamic spring constant was measured with a preload of 3,510 N, afrequency of 100 Hz and an amplitude of ±0.05 mm. The absolute springconstant was measured with a preload of 3,510 N, a frequency of 300 Hzand an amplitude of 5 G (fixed).

As is apparent from Table 2, the strut mounts obtained from the rubbercompositions in Example 1 and 2 had the low dynamic spring constant andthe low absolute spring constant compared to the strut mount obtainedfrom the rubber composition in Comparative Example 1. Therefore, thedynamic factor was low with respect to the ratio of the dynamic springconstant to the static spring constant and the ratio of the absolutespring constant to the static spring constant. The order of the dynamicfactor of the test pieces shown in Table 1 (Example 2<Example1<<Comparative Example 1) was maintained also in the dynamic factor(dynamic spring constant and absolute spring constant) in the strutmounts shown in Table 2. Accordingly, it was found that in Examples 3 to9, as in Examples 1 and 2, the effect of the decrease in the dynamicfactor was high when using the strut mounts.

The strut mounts obtained from the rubber compositions in Example 2 andComparative Example 1 were installed in an automobile, and an actualautomobile test was conducted to identify the effect of the decrease inthe road noise.

A sound level at a speed of 60 km/hour was measured using an FrontEngine Front Drive car of 1,500 cc. A microphone was placed near the earon the side of the front sheet of the car. The results are shown inTable 4.

A general tire and a light-weight tire shown in Table 3 were used as atire mounted on the car. The size of these two tires was 175/70R13 82S.The half sections of the two tires are shown in FIG. 3. That is, thesection on the left side is a section of a general tire 80, and thesection on the right side is a section of a light-weight tire 90. Asshown in FIG. 3, in the general tire 80, a carcass 81 is made of 1 ply;it is much folded from a bead 82 and is turned up to a center of a sideportion 83. The thicknesses in normal directions of the tread 84, theside portion 83 and the bead 82 are A=15.0 mm, B=5.0 mm and C=10.0 mmrespectively. On the other hand, in the light-weight tire 90, a carcass91 is made of 1 ply; it is slightly folded in a bead 92, and the foldingis terminated at a lower portion of the bead 92. The thicknesses innormal directions of the tread 93, the side portion 94 and the bead 92are A=13.5 mm, B=4.0 mm and C=8.5 mm respectively. The thicknesses ofthe light-weight tire 90 are smaller than those of the general tire 80by from 10 to 20%. Consequently, as shown in Table 3, the weight of thelight-weight tire 90 is reduced by 10% compared to the weight of thegeneral tire 80.

Further, the coefficient of the rolling resistance (index) of thelight-weight tire 90 is, as shown in Table 3, 65, whereas that of thegeneral tire 80 is 100. Thus, the light-weight tire 90 is a low rollingresistance tire. Incidentally, both of the tires are equal with respectto a radial spring constant, a transverse spring constant, a corneringpower and a self-aligning torque.

                  TABLE 3                                                         ______________________________________                                                         Tire                                                                            General Light-weight                                       Item               tire    tire                                               ______________________________________                                        Weight of a tire (kg)                                                                            6.6     5.5                                                Radial spring constant (N/mm)                                                                    162     160                                                Transverse spring constant                                                                       89      75                                                 (N/mm)                                                                        Cornering power (kN/deg)                                                                         0.677   0.677                                              Self-aligning torque (N · m)                                                            30      34                                                 Thickness of a tread (mm) A                                                                      15.0    13.5                                               Thickness of a side portion                                                                      5.0     4.0                                                (mm) B                                                                        Thickness of a bead (mm) C                                                                       10.0    8.5                                                Coefficient of rolling                                                                           100     65                                                 resistance (index)                                                            ______________________________________                                    

A rim used was 13×51/2-JJ, and an air pressure was 200 kPa (2.0kgf/cm²).

As shown in FIG. 4, when the light-weight tires were mounted on the carhaving the strut mounts in Comparative Example 1, the sound pressurelevel was increased by 1 dB near 160 Hz and 250 Hz compared to the casewhere the general tires were mounted thereon. On the other hand, whenthe light-weight tires were mounted on the car having the strut mountsin Example 2, the sound pressure level was decreased by 2 dB near 160 Hzand 250 Hz compared to the case where the general tires were mounted onthe car having the strut mounts in Comparative Example 1. Further, inthe evaluation of sensitivity as well, the effect of the improvement inthe road noise was identified.

Thus, when the light-weight tires were mounted on the car, the roadnoise was decreased upon using the strut mounts in Example 2.Accordingly, the light-weight tires can preferably be used.

What we claim is:
 1. A vibration-isolating rubber assembly, comprising avibration-isolating rubber member and a metal member, in which a dynamicfactor is not larger than about 1.7, said dynamic factor being definedas a ratio of dynamic spring constant to static spring constant.
 2. Thevibration-isolating rubber assembly of claim 1, wherein said dynamicfactor is not larger than about 1.6.
 3. The vibration-isolating rubberassembly of claim 1, wherein-said vibration-isolating rubber member iscomprised of a polybutadiene rubber polymerized in the presence of aneodymium catalyst.
 4. The vibration-isolating rubber assembly of claim2, wherein-said vibration-isolating rubber member is comprised of apolybutadiene rubber polymerized in the presence of a neodymiumcatalyst.
 5. The vibration-isolating rubber assembly of claim 1, whereinsaid vibration-isolating rubber member is comprised of a polybutadienerubber which has a cis content of at least about 98%.
 6. Thevibration-isolating rubber assembly of claim 2, wherein saidvibration-isolating rubber member is comprised of a polybutadiene rubberwhich has a cis content of at least about 98%.
 7. Thevibration-isolating rubber assembly of claim 1, wherein saidvibration-isolating rubber member contains a carbon black having anitrogen adsorption specific surface area of no greater than 40m² /g anda dibutyl phthalate oil absorption of at least 80 ml/100 g.
 8. Thevibration-isolating rubber assembly of claim 2, wherein saidvibration-isolating rubber member contains a carbon black having anitrogen adsorption specific surface area of no greater than 40m² /g anda dibutyl phthalate oil absorption of at least 80 ml/100 g.
 9. Thevibration-isolating rubber assembly according to claim 1, wherein saidvibration-isolating rubber member contains a compound represented by aformula:

    RNH(CH.sub.2).sub.n NHR

wherein:R represents: ##STR2## n is between 2 and
 12. 10. Thevibration-isolating rubber assembly according to claim 2, wherein saidvibration-isolating rubber member contains a compound represented by aformula:

    RNH(CH.sub.2).sub.n NHR

wherein:R represents: ##STR3## n is between 2 and
 12. 11. Thevibration-isolating rubber assembly according to claim 1, wherein saidvibration-isolating rubber assembly comprises one of a rubber bush, acylindrical bush, a mount, a support, and a cradle mount.
 12. Thevibration-isolating rubber assembly according to claim 2, wherein saidvibration-isolating rubber assembly comprises one of a rubber bush, acylindrical bush, a mount, a support, and a cradle mount.